Centrifugal turbine compressors have been used for many years to raise the pressure of natural gas or other compressible fluids in pipelines. By increasing the pipeline pressure of such fluids a corresponding increase in the mass flowrate of the fluid through the pipeline occurs, resulting in a greater quantity of fluid being delivered to its desired destination within a shorter period of time than would occur at lower pressure values.
It is generally known that a given compressor has a unique operating relationship to the pressure and flowrate of the compressible fluid. This relationship depends, to a large extent, on the internal staging and geometry of the compressor. However, all centrifugal compressors follow a general performance pattern. For a fixed operating speed, the flowrate of the fluid being compressed will decrease as the head pressure or pressure rise through the compressor increases. A compressor will operate in this fashion until it reaches its surge limit. The surge limit is reached when the compressor stalls producing a flow reversal of the fluid being compressed. Operation of the compressor as near as possible to the surge limit, at a fixed compressor speed, is highly desirable because at the surge limit the pressure of the fluid is maximum and, consequently, compressor operation is maximally efficient.
This previously described flow reversal produces a small decrease in the fluid discharge pressure of the compressor. As a result of the decrease in compressor discharge pressure, the fluid flowrate increases, thereby causing an increase in the compressor speed. As a result, the compressor is forced back to its former fixed operating speed. The compressor will maintain its previous fixed operating speed until the pressure of the compressed fluid, once again, rises to the surge limit, after which the entire process, as previously described, is repeated.
This cyclic instability is simply referred to as surge. The frequency of its occurrence as the compressor is operated near the surge limit will typically vary from one-half a cycle to five cycles per second.
Initially, these flow reverses are harmless because of their reduced magnitude and low frequency of occurrence. However, as the magnitude and frequency of the flow reversals increase, the effect upon the compressor being operated close to the surge limit is destructive. When such a surge condition occurs, it produces concurrent axial displacements of the compressor rotor and the rotor shaft. These rapid axial displacements of the rotor shaft cause the shaft to oscillate back and forth between the rotor shaft bearings until the bearings are damaged. Once rotor bearing failure occurs, rotation of the rotor ceases and the compressor is rendered inoperative.
Consequently, in order to operate the compressor as efficiently as possible while avoiding destruction of the compressor assembly, it is necessary to monitor the various operational parameters of the compressor, such as compressor discharge pressure or compressor discharge temperature, to determine when the surge limit of the compressor has been reached.
One method employed in the prior art to detect the surge limit was to select an orifice, the size of which is determined by the known flow-pressure characteristics of the compressor to be monitored, and to insert this orifice into the fluid flow stream. Then, by measuring the fluid pressure differential measuring the fluid pressure differential across the orifice, the actual flowrate in the system could be determined using the well-known mathematical relationship established by Bernoulli between fluid pressure and flowrate.
However, this method is undesirable for a number of reasons. One reason is that the placement of an orifice directly in the fluid flow stream produces a pressure drop in the pipeline and reduces fluid flowrate. The effect is a reduction in the over-all efficiency of the function of the compressor-pipeline system. Another reason is that the introduction of such an orifice into the fluid flow stream creates an artificial compressor surge limit. One effect of such an artificial surge limit is to lower the maximum fluid pressure obtainable at a fixed compressor operating speed. Further, because it is an artificial, rather than the actual surge limit of the compressor, subsequent changes in the pipeline and compressor operating conditions can render the system useless by causing the artificial surge limit to shift, necessitating an expensive re-evaluation of the system's operating conditions.
Another method measures the compressor's fluid discharge temperature. However, this method is undesirable because it requires expensive electronic equipment and is highly sensitive to the deleterious effects of cavitation caused by the immersion of the temperature sensor in the fluid flowstream. Such fluid immersion not only results in a short life-expectancy for the temperature sensor but also typically produces inaccuracies due to the rapidly changing nature of the fluid and the flow conditions.